JP4894282B2 - Electric double layer capacitor - Google Patents

Abstract

An electric double layer capacitor includes a first collector, a first polarizable electrode layer provided on the first collector, a second collector, a second polarizable electrode layer provided on the second collector and facing the first polarizable electrode layer, a separator having an insulating property and provided between the first polarizable electrode layer and the second polarizable electrode layer, and a driving electrolyte impregnated in the first polarizable electrode layer and the second polarizable electrode layer. The polarizable electrode layers mainly contain activated carbon made from phenol resin, have a surface roughness not larger than 0.6µm, and have an electrode density ranging from 0.5 g/cm 3 to 0.7 g/cm 3 This electric double layer capacitor has characteristics deteriorating little even at low temperatures, thus having a reliability for a long time.

Description

  The present invention relates to an electric double layer capacitor used in various electronic devices.

  A conventional electric double layer capacitor of this type is configured to have a capacitor element by facing a polarizable electrode on the anode side and a polarizable electrode on the cathode side with a separator interposed therebetween, and is driven by this capacitor element. It is comprised by impregnating the electrolyte solution for use.

  And as said polarizable electrode, what provided the activated carbon electrode layer is generally used, and functional groups, such as a carboxyl group, a phenolic hydroxyl group, a carbonyl group, and a quinone group, exist on the surface of this activated carbon, It is known that these surface functional groups have a great influence on the characteristics of the electric double layer capacitor. In particular, an electric double layer capacitor having an activated carbon electrode layer with a large amount of acidic surface functional groups (carboxyl group, carbonyl group, phenolic hydroxyl group, lactone group) is increased by increasing the capacity site contributing to the electric double layer capacity. It is known that the pseudo capacity increases.

  In addition, there exists a technique which is proposed by patent document 1 as what paid attention to the amount of acidic surface functional groups of such an activated carbon electrode, In this technique, the group of a carboxyl group, a quinone group, a hydroxyl group, and a lactone group is mentioned. By specifying the total amount of at least one kind of surface functional group selected from 0.2 to 1.00 milliequivalent per 1 g of activated carbon, a high capacitance can be obtained. As the electrolytic solution, a quaternary ammonium salt is used in an aprotic polar solvent such as propylene carbonate.

  Moreover, in patent document 2 and patent document 3, by using what melt | dissolved amidine salt in aprotic polar solvents, such as a propylene carbonate, as a drive electrolyte, compared with the case where a quaternary ammonium salt is used. However, in these cases, attention has not been paid to the low-temperature characteristics, and only the results at the room temperature characteristics are described.

JP 2000-169129 A JP 11-54376 A Japanese Patent Laid-Open No. 11-54379

  However, in the above-mentioned conventional electric double layer capacitor, a driving electrolyte is prepared by dissolving an amidine salt in an aprotic polar solvent such as propylene carbonate, and activated carbon having a large amount of acidic surface functional groups is used for the polarizable electrode layer. When used, the pseudo capacitance of the electric double layer capacitor can be increased, but during the charge / discharge reliability test, the driving electrolyte generates an electrochemical reaction with the acidic surface functional group on the surface of the polarizable electrode layer. Deteriorated substances, etc. are increased, and the deteriorated substances block the activated carbon capacity site, resulting in a large deterioration in characteristics such as capacity deterioration and resistance increase. there were.

  An object of the present invention is to solve such a conventional problem, and to provide an electric double layer capacitor with little deterioration in characteristics and excellent in low temperature characteristics and long-term reliability.

In order to solve the above problems, the present invention was formed by winding or laminating a pair of positive and negative electrodes, each having a polarizable electrode layer formed on a current collector made of a metal foil, with a separator interposed therebetween. An electric double layer capacitor comprising at least a capacitor element and a driving electrolyte impregnated in the capacitor element, wherein the polarizable electrode layer includes activated carbon made of a phenol resin , a conductive material, and a binder, and the polarizability 8% by weight or less of the conductive material and 2-8% by weight of the binder are added to the electrode layer , the activated carbon has an average particle diameter of 2.5 to 3.5 μm, and the surface roughness of the polarizable electrode layer The thickness Ra is 0.2 to 0.6 μm, and the electrode density is 0.5 to 0.7 g / cm ³ .

  Furthermore, the amount of acidic surface functional groups of activated carbon made of a phenol resin forming the polarizable electrode layer is 0.31 to 0.37 milliequivalent per 1 g of activated carbon, and the driving electrolyte is aprotic of propylene carbonate. The amidine salt is dissolved in a polar solvent.

  As described above, the electric double layer capacitor according to the present invention uses the activated carbon optimized for the surface roughness and the electrode density of the polarizable electrode layer to perform the electrochemical reaction on the surface of the polarizable electrode without reducing the capacity. Therefore, the electric double layer capacitor having a large capacity, little characteristic deterioration, excellent low-temperature characteristics and long-term reliability can be realized.

  In addition, by using activated carbon with an optimized amount of acidic surface functional groups, it is possible to shift to the optimal potential window of a driving electrolyte solution in which an amidine salt is dissolved in an aprotic polar solvent of propylene carbonate. The effect that improvement can be aimed at is obtained.

(Embodiment 1)
Hereinafter, the first aspect of the present invention will be described with reference to the first embodiment.

  FIG. 1 is a partially cutaway perspective view showing the configuration of an electric double layer capacitor according to Embodiment 1 of the present invention, and FIG. 2 is a cross-sectional view showing a polarizable electrode body used in the electric double layer capacitor. 1 and 2, reference numeral 1 denotes a capacitor element. The capacitor element 1 includes an anode-side polarizable electrode body 3 to which an anode lead wire 2 is connected and a cathode-side polarizable electrode to which a cathode lead wire 4 is connected. The body 5 is formed by winding a separator 6 for preventing a short circuit therebetween.

  Also, the polarizable electrode body 3 on the anode side is coated with a polarizable electrode layer 3b containing activated carbon having an acidic surface functional group on the surface of a current collector 3a made of an aluminum foil roughened in an electrolytic solution. Although the description is omitted, the polarizable electrode body 5 on the cathode side is also formed in the same manner.

  7 is a rubber sealing member provided with a hole through which the anode lead wire 2 and the cathode lead wire 4 are inserted, and 8 is impregnated with a driving electrolyte (not shown) in the capacitor element 1. The bottomed cylindrical metal case is made of aluminum and is stored in a compressed state. By drawing and curling the opening of the metal case 8, the sealing member 7 is compressed to seal the metal case 8. It is comprised so that it may do.

  Here, by pressing the polarizable electrode layer 3b constituting the polarizable electrode body 3 on the anode side, the surface roughness of the polarizable electrode layer 3b is reduced and the electrode density is increased. A plurality of samples having different surface roughness and electrode density of the polarizable electrode layer 3b were produced by changing the conditions. The cathode-side polarizable electrode body 5 was similarly prepared and evaluated, but only the anode-side polarizable electrode body 3 will be described here.

Then, the surface roughness Ra (μm) and the electrode density (g / cm ³ ) of the polarizable electrode layer 3b of the polarizable electrode body 3 thus manufactured and the initial resistance value at a low temperature (−30 ° C.) are measured. The results of evaluating these relationships are shown in FIG. 3 (a). Further, after applying 2.5V to the sample in a 60 ° C. atmosphere for 1000 hours, the resistance value at a low temperature (−30 ° C.) was measured, The result of evaluating the rate of change with respect to the initial resistance value is shown in FIG.

  In addition, the measurement of the said surface roughness was performed using the shape measurement microscope and VK8500 by a KEYENCE company.

  As is clear from FIGS. 3A and 3B, when the surface roughness Ra is 0.6 μm or less, the resistance value at a low temperature (−30 ° C.) is as low as less than 150 mΩ, and the change with respect to the initial resistance value is as follows. It can be seen that the rate is also small.

  From these results, it is presumed that the resistance value at a low temperature (−30 ° C.) decreases as the surface roughness Ra approaches zero as much as possible and the electrode density is increased. In view of the strength problem of the current collector 3a constituting 3 and the press apparatus used in the experiment, it was not possible to confirm that the surface roughness Ra was less than about 0.2 μm.

In addition, the surface roughness Ra of the polarizable electrode layer 3b, which is judged to be excellent because the resistance value at the low temperature (−30 ° C.) is low and the rate of change with respect to the initial resistance value is small, is 0.6 μm or less. As shown in FIGS. 3A and 3B, the electrode density is found to be approximately 0.5 to 0.7 g / cm ³ .

As described above, the electric double layer capacitor according to the present embodiment has a surface roughness Ra of 0.6 μm or less and an electrode density of the polarizable electrode layer that is formed on the current collector and constitutes the polarizable electrode body. An electric double layer capacitor having a low temperature characteristic and a long-term reliability by reducing the resistance value at a low temperature (−30 ° C.) and keeping the rate of change small by setting 0.5 to 0.7 g / cm ^3. It will be able to provide.

(Embodiment 2)
The second aspect of the present invention will be described below with reference to the second embodiment.

  The present embodiment changes the acidic surface functional group amount of activated carbon made of phenol resin that forms the polarizable electrode layer of the electric double layer capacitor described in the first embodiment, and is combined with the driving electrolyte. Since the configuration other than that is the same as that of the first embodiment, the same reference numerals are given to the same portions and the detailed description thereof is omitted, and only different portions will be described below.

  The amount of acidic surface functional groups (carboxyl group, carbonyl group, phenolic hydroxyl group, lactone group) of activated carbon used for the anode side polarizable electrode body 3 and the cathode side polarizable electrode body 5 is set to a heat treatment temperature of 800 ° C. or higher. As shown in (Table 1) by combining these with the driving electrolyte solution, as shown in Table 1, Examples 1 to 3 according to this embodiment and Conventional Examples 1 to 7 as comparative examples are used. A wound type electric double layer capacitor (rated voltage 2.0 V—capacitance 68 F, size: φ18 mm × L50 mm) was produced. In addition, the said acidic surface functional group amount is measured using a reverse titration method.

  And after performing the aging which applies the voltage of 2.0V to these electric double layer capacitors for 12 hours continuously at 60 degreeC, the constant current constant voltage charge of 1.5A and 2.0V was performed, and after charge The capacity (discharge capacity) and internal resistance at 1.0 A discharge were measured. Moreover, about these electric double layer capacitors, the voltage of 2.3V was applied continuously at 60 degreeC, and the capacity | capacitance and internal resistance of the capacitor 1000 hours after that were measured with the measuring method similar to the above. The results of the room temperature characteristics are shown in (Table 2), and the results of the low temperature characteristics are shown in (Table 3).

  As is clear from (Table 2) and (Table 3), the characteristics of the electric double layer capacitors composed of the quaternary ammonium salts of Conventional Examples 1 to 5 are the same as those of Conventional Examples 6 and 7 and Examples 1 to 3. It turns out that it is inferior to the characteristic of the electric double layer capacitor comprised by the amidine salt.

  In addition, the electric double layer capacitor constructed according to Conventional Example 7 has a higher initial capacity and a pseudo capacity than the result of the room temperature characteristics of (Table 2), but the capacity after 1000 hours of the reliability test is an example. The electric double layer capacitor constituted by 1 and 2 is lower than the capacity after 1000 hours of reliability test and also has high internal resistance. Furthermore, it can be seen from the results of the low temperature test of (Table 3) that the initial characteristics and the characteristics after 1000 hours of the reliability test are inferior to the same characteristics of the electric double layer capacitors configured according to Examples 1 and 2.

  In addition, it can be seen from Tables 2 and 3 that the electric double layer capacitor constructed according to Conventional Example 6 has a large decrease in the initial capacity at room temperature and low temperature because the amount of the acidic surface functional group is too small. .

  Moreover, the electric double layer capacitor comprised by Example 1, 2 is substantially equivalent to the initial characteristic of the electric double layer capacitor comprised by Example 3 and the prior art example 7 from the result of the normal temperature characteristic of (Table 2). It has the maximum capacity and good characteristics after 1000 hours of reliability test. From the results of the low temperature characteristics shown in (Table 3), the initial capacity of the electric double layer capacitors constructed according to Examples 1 and 2 is the highest, and the characteristics after 1000 hours of the reliability test are also good.

  From the above results, although the electric double layer capacitor constructed according to Example 3 and Conventional Example 7 has a high initial capacity, on the other hand, the internal resistance is large, and the change rate of the capacity and resistance from the initial stage is large. It lacks long-term reliability. On the contrary, the electric double layer capacitor constituted by the prior art example 6 has excellent long-term reliability because of the small change rate of the capacity and resistance from the initial stage, but on the other hand, the initial capacity drop is too large. On the other hand, according to the electric double layer capacitor configured according to Examples 1 and 2, there is no disadvantage as in the electric double layer capacitor configured according to Example 3 and Conventional Examples 6 and 7, and the maximum capacity is obtained. It can be seen that long-term reliability improvements can be achieved simultaneously.

  Therefore, in an electric double layer capacitor using a driving electrolyte solution in which an amidine salt is dissolved in an aprotic polar solvent such as propylene carbonate, a raw material of activated carbon which is a constituent material of a polarizable electrode body is a phenol resin. It can be said that the acidic surface functional group amount of the activated carbon is preferably in the range of 0.31 to 0.37 milliequivalent per 1 g of activated carbon.

  By adopting such a configuration, the electrochemical reaction between the driving electrolyte and the acidic surface functional group on the surface of the polarizable electrode can be suppressed, and an amidine salt can be added to the aprotic polar solvent of propylene carbonate. Since the breakdown voltage can be improved by shifting to the optimal potential window of the driving electrolyte in which the electrolyte is dissolved, gas generation, resistance increase, and capacitance change due to the occurrence of electrochemical reactions are eliminated, which leads to deterioration of characteristics. Therefore, it is possible to provide an electric double layer capacitor having a low temperature characteristic and excellent long-term reliability.

(Embodiment 3)
The third embodiment of the present invention will be described below in particular.

  In the present embodiment, the configuration of the activated carbon made of phenol resin that forms the polarizable electrode layer of the electric double layer capacitor described in the second embodiment is further limited, and other configurations are the same as those in the second embodiment. Therefore, the same parts are denoted by the same reference numerals, and detailed description thereof is omitted. Only different parts will be described below.

  In the present embodiment, the capacity density per volume of the phenol resin activated carbon that is a raw material of the activated carbon can be kept to the maximum. When the specific surface area of the activated carbon is increased, the activated carbon is proportionally increased. Although the per unit volume can be increased, the pore volume also increases at the same time, resulting in a slight activated carbon, and the per unit volume decreases. The expression of the electric double layer capacity greatly depends on the pore distribution.

Therefore, by designing the pore volume capable of efficiently expressing the capacity by suppressing the pores that are wasted, the volume is set to 1.1 cm ³ / g or less, and the specific surface area is set to 2000 m ² / g or more. The capacity density per volume of the phenol resin activated carbon that is the raw material of the activated carbon can be kept to the maximum.

  In this way, the capacity reduction caused by the degradation material generated by the electrochemical reaction of the driving electrolyte solution with the acidic surface functional group on the surface of the polarizable electrode during the charge / discharge reliability test blocks the capacity site of the activated carbon. Thus, the influence of characteristic deterioration such as an increase in resistance can be minimized.

(Embodiment 4)
The fourth embodiment of the present invention will be described below with reference to the fourth embodiment.

  In the present embodiment, the configuration of the activated carbon made of phenol resin that forms the polarizable electrode layer of the electric double layer capacitor described in the second embodiment is further limited, and other configurations are the same as those in the second embodiment. Therefore, the same parts are denoted by the same reference numerals, and detailed description thereof is omitted. Only different parts will be described below.

  In the present embodiment, the capacity density per volume of the phenol resin activated carbon that is a raw material of the activated carbon is kept to the maximum, and the resistance of the electric double layer capacitor can be reduced. When the particle size is increased, the diffusion contribution of the electrolyte ions diffusing in the activated carbon pores in the polarizable electrode layer is increased and the resistance is increased, and this increase in diffusion resistance makes it impossible to draw capacity during charging and discharging. If the average particle size of the activated carbon is too small, the contribution of the contact resistance between the activated carbon particles and the interface resistance between the activated carbon particles and the current collector increases in the polarizable electrode layer, and the resistance increases. Due to the increase in resistance and interface resistance, it becomes impossible to draw capacity during charging and discharging.

  Here, the relationship between the volume capacity density and the resistance according to the average particle diameter of the activated carbon was measured, and the results are shown in (Table 4) and FIGS. 4 shows the relationship between the activated carbon average particle size and volumetric capacity density at 25 ° C., FIG. 5 shows the relationship between the activated carbon average particle size and resistance at 25 ° C., and FIG. 6 shows the activated carbon average particle size and volume at −30 ° C. FIG. 7 shows the relationship between the capacity density and the relationship between the activated carbon average particle diameter at −30 ° C. and the resistance.

  As can be seen from Table 4 and FIGS. 4 to 7, by setting the average particle size of the activated carbon in the range of 2.5 to 3.5 μm, the capacity density per volume of the phenol resin activated carbon that is the raw material of the activated carbon. And the resistance of the electric double layer capacitor can be reduced. In particular, the resistance reduction at a low temperature becomes remarkable.

  Therefore, when an electrolyte solution for driving in which an amidine salt is dissolved in an aprotic polar solvent such as propylene carbonate is used, the average particle diameter of the phenol resin activated carbon that is a constituent material of the polarizable electrode body is 2.5. It can be said that a range of ˜3.5 μm is suitable.

  FIG. 8 shows the relationship between the hardness (degree) of the average particle diameter of the activated carbon and the resistance at −30 ° C., and the average particle diameter of the phenol resin activated carbon is considered to be 2.5. It can be seen that the hardness in the range of ˜3.5 μm is approximately 81 to 83 degrees.

(Embodiment 5)
Hereinafter, the invention according to claim 5 of the present invention will be described with reference to the fifth embodiment.

  In the present embodiment, the configuration of the polarizable electrode layer of the electric double layer capacitor described in the second embodiment is partially different, and other configurations are the same as those in the second embodiment. The same parts are denoted by the same reference numerals and detailed description thereof is omitted, and only different parts will be described below.

  In this embodiment, the amount of the conductive material contained in the activated carbon layer of the polarizable electrode body 3 on the anode side and the activated carbon layer of the polarizable electrode body 5 on the cathode side is the same as that of the activated carbon layer of the polarizable electrode body. 8% or less of the material, and further, the binder amount is 2 to 8%. This makes it possible to reduce the contact resistance and greatly improve the low temperature characteristics. is there. The binder amount is more preferably in the range of 4 to 6%.

  Thus, by optimizing the activated carbon layer of the polarizable electrode body, it is possible to suppress deterioration of capacity and increase in resistance.

  The electric double layer capacitor according to the present invention has a large capacity, has an effect of being excellent in long-term reliability, and has a long life, power density and low temperature characteristics. It is useful as an electric double layer capacitor for a hybrid vehicle that requires improvement.

1 is a partially cutaway perspective view showing a configuration of an electric double layer capacitor according to Embodiment 1 of the present invention. Sectional drawing which showed the structure of the polarizable electrode body used for the same electric double layer capacitor (A) Characteristic diagram showing the relationship between the electrode density depending on the surface roughness of the polarizable electrode layer constituting the polarizable electrode body and the initial resistance value at −30 ° C., (b) Change in resistance with respect to the initial resistance value Characteristic chart showing rate Characteristic chart showing the relationship between activated carbon average particle size and volume capacity density at 25 ° C Characteristics chart showing the relationship between activated carbon average particle size and resistance at 25 ℃ Characteristic diagram showing the relationship between activated carbon average particle size and volume capacity density at -30 ℃ Characteristic diagram showing the relationship between activated carbon average particle size and resistance at -30 ℃ Characteristic diagram showing the relationship between hardness and resistance depending on the average particle diameter of activated carbon at -30 ℃

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Capacitor element 2 Anode lead wire 3 Anode-side polarizable electrode body 3a Current collector 3b Polarized electrode body Activated carbon layer 4 Cathode lead wire 5 Cathode-side polarizable electrode body 6 Separator 7 Sealing member 8 Metal case

Claims (3)

A capacitor element formed by winding or laminating a pair of positive and negative electrodes having a polarizable electrode layer formed on a current collector made of a metal foil with a separator interposed therebetween, and the capacitor element was impregnated An electric double layer capacitor comprising at least a driving electrolyte , wherein the polarizable electrode layer includes activated carbon made of a phenol resin, a conductive material, and a binder, and 8 mass of the conductive material with respect to the polarizable electrode layer. %, The binder is added in an amount of 2 to 8% by mass, the activated carbon has an average particle size of 2.5 to 3.5 μm , the surface roughness Ra of the polarizable electrode layer is 0.2 to 0.6 μm, and An electric double layer capacitor having an electrode density of 0.5 to 0.7 g / cm ³ . The amount of acidic surface functional groups of activated carbon made of phenol resin that forms a polarizable electrode layer is 0.31 to 0.37 milliequivalent per 1 g of activated carbon, and the driving electrolyte is an aprotic polar solvent of propylene carbonate The electric double layer capacitor according to claim 1, wherein an amidine salt is dissolved in the electric double layer capacitor. ^2. The electric double layer capacitor according to claim 1, wherein the activated carbon composed of a phenol resin forming the polarizable electrode layer has a specific surface area of 2000 m ² / g or more and a pore volume of 1.1 cm ³ / g or less .

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